1. Technical Field
The present invention relates generally to photoresists used as a mask for etching purposes, and more particularly to a method of forming a hardened layer on the exposed resist surfaces to slow the rate of erosion during etching procedures.
2. Background Art
The process of accurately etching patterns has been the subject of significant development, particularly in the field of semiconductor electronics. The degree of circuit miniaturization, affecting a product's size and operational frequency limits for example, depend on the degree of accuracy of etching. The process of etching a pattern involves the use of a mask to selectively allow an etchant to remove the semiconductor or conductive material, as required to form the desired pattern. Typically, a mask is formed by spin coating a layer of liquid photoresist on the material to be etched. The desired pattern on the photoresist is then exposed to a form of radiation, for example, through use of an optical mask and ultraviolet light. The exposed areas of the photoresist are rendered either soluble or insoluble to a developer, depending on whether the resist is a positive or negative type. The soluble portions are then removed, and the remaining photoresist functions as a mask for selectively allowing an etchant to remove underlying material in areas void of photoresist protection.
An ideal etchant would not attack the photoresist, or any material underlining the material to be removed, such as semiconductor material below conductive film, or a second semiconductor material under a first semiconductor material. The ideal etchant would also be anisotropic, i.e. etch only straight down into the material to be etched, and not etch laterally. Generally, all purely chemical etchants are isotropic, etching equally in both lateral and vertical directions. This is generally an undesirable characteristic, as illustrated in FIGS. 1a and 1b. FIG. 1a is a simple cross-sectional view of a resist pattern 10 of width w, deposited on a first material 12 to the etched, which is deposited on top of a second material 14. FIG. 1a is before application of an etchant. FIG. 1b shows the resist cross-section 10 reduced in width by 2d. This reduction causes the patterned, etched second material 12 to also be a width less than w by 2d. Actually, FIGS. 1a and 1b ignore the fact that the resist 10 and material to be etched 12, normally etch at different rates. This is a serious problem when isotropic etchants such as chemicals are used, which etch in all directions. The result of using chemical etchants is illustrated in FIGS. 2a and 2b where the resist 16 width w is reduced by 2d1 and the material 18 to be etched is reduced in width by an additional amount 2d2. Since the chemical etchant is isotropic, and designed to attack the material to be etched at a faster rate than the photoresist, a chemical etchant usually undercuts the photoresist to some degree, as shown at 19 in FIG. 2b.
Due to the isotropic nature of chemical etchants, other methods of etching have evolved. One such method is sputtering, which is highly directional, i.e. anisotropic. Unfortunately, this type of etching is not selective, attacking the photoresist as well as the material to be removed and underlying material. Sputtering also causes redeposition of non-volatile species on the side walls of the etched feature. As a result of these problems, this method is not highly successful. An improved method of etching involves a combination of chemical etching, along with impacting the material with energized ions. Regardless of the type of etchant used, the resist is a critical element. Ideally, the resist would be very thin, and would etch very slowly relative to the exposed material to be etched. In order to create a thin, "hard mask", a more complicated process is sometimes used, as shown in FIGS. 3a and 3b, wherein a first layer of hard material 16, such as Si.sub.3 N.sub.4 is deposited, for example over a SiO.sub.2 layer 18 to be etched, which is in turn deposited on a Si substrate 20. A layer of conventional resist 22 is then deposited over the Si.sub.3 N.sub.4 16. The objective is to deposit enough resist 22 so that the etchant will erode through the exposed Si.sub.3 N.sub.4 16 and SiO.sub.2 18 before the etchant severely erodes the Si.sub.3 N.sub.4 16 areas underlying the resist 22. The resultant etched situation before resist 22 and Si.sub.3 N.sub.4 16 removal is illustrated in FIG. 3b showing the resist 22 highly eroded, and the Si.sub.3 N.sub.4 16 slightly eroded but the SiO.sub.2 18 underlying still intact. Following this etching procedure, the Si.sub.3 N.sub.4 16 and resist 22 must be removed. This approach provides some improvement over the other methods discussed, but has the disadvantage of being very complicated.
There is a need for an improved resist material that is less susceptible to erosion by etchants, providing improved accuracy in etching in order to expand the frequency range and reduce dimensions of circuits.